1. Field of the Invention
The present invention relates to a method for measuring creatine kinase activity. More particularly, it relates to a method employing a composition containing phosphoglycerate kinase (PGK) and glyceraldehyde-3-phosphate dehydrogenase (GAPDH).
2. Description of the Prior Art
The activity of CK is one of the indices of clinical diagnosis, for example, progressive muscular dystrophy and myocardial infarction. Hitherto, CK activity has generally been measured using as a coupling enzyme for CK a reagent comprising hexokinase and glucose-6-phosphate dehydrogenase as main components (hereinafter referred to as "HK method"). The HK method is based on the following equations (1), (4) and (5). The enzymes which catalyze the reactions of the equations (4) and (5) are hexokinase and glucose-6-phosphate dehydrogenase, respectively. EQU Creatinephosphate+ADP.revreaction.Creatine+ATP (1) EQU ATP+Glucose.revreaction.ADP+Glucose-6-phosphate (4) EQU Glucose-6-phosphate+NADP.revreaction.6-phosphogluconate+NADPH (5)
While the HK method is fairly accurate, the storage stability of hexokinase or glucose-6-phosphate dehydrogenase is inferior and the reagent for the measurement of CK activity can be kept stable only for several hours after preparation. Accordingly, it has been desired to provide coupling enzymes having more excellent storage stability.
Further, in the HK method, reduced glutathione (hereinafter referred to as "GSH") is preferably used. However, when a glutathione reductase (hereinafter referred to as "GR") is present in the test sample, GSH oxidized by some reaction or other is reduced and, at the same time, reduced .beta.-nicotinamideadenine dinucleotide phosphate (hereinafter referred to as "NADPH") formed by a primary reaction is reoxidized to reform oxidative nicotinamideadenine dinucleotide phosphate (hereinafter referred to as "NADP"). Consequently, there is a danger of misjudging the CK activity in the test samples as being low.
As a result of earnest studies directed to improving the above-described disadvantages in the prior method of the measurement, it has been found that if a composition having excellent storage stability comprising PGK and GAPDH as main components is used as a coupling enzyme for measurement of the CK activity, accurate measurement can be carried out without a great influence by GR in the test sample, and thus the present invention has been established.